Production of proteins and polypeptides from DNA can be achieved in various hosts, but a common problem is the formation of insoluble protein/polypeptide aggregates. This may severely impede or even prevent production of a functional protein/polypeptide. One solution to this problem is to express the desired protein or polypeptide as a fusion protein with a protein or polypeptide that provides the required solubility. The fusion protein may be cleaved, and the desired protein isolated.
The problem is typically aggravated with low-solubility proteins and polypeptides, e.g. membrane-associated proteins and polypeptides. For instance, lung surfactant protein C (SP-C; Table 6) is a transmembrane protein that is produced by alveolar type II cells and is a constituent of surfactant, that is necessary to prevent alveolar collapse at end expiration. Neonatals often suffer from respiratory distress due to insufficient amounts of surfactant. Today, this condition is treated with surfactant preparations extracted from animal lungs. SP-C-33 is a variant of SP-C, where the residues in the transmembrane helix (normally mainly valines) are exchanged for leucines. SP-C-33 retains the function of native SP-C, including proper insertion in membranes, but is less prone to aggregate and therefore feasible to produce in large quantities for development of a synthetic surfactant preparation. Since SP-C-33 so far has not been possible to produce from DNA, it is today manufactured by chemical synthesis.
Other examples of proteins and polypeptides that pose difficulties when expressed from recombinant DNA are Aβ-peptide, IAPP, PrP, α-synuclein, calcitonin, prolactin, cystatin, ATF and actin; SP-B, α-defensins and β-defensins; class A-H apolipoproteins; LL-37, SP-C, SP-C33Leu, Brichos, GFP, neuroserpin; hormones, including EPO and GH, and growth factors, including IGF-I and IGF-II; avidin and streptavidin; and protease 3C.